1. Field of the Invention
The invention relates generally to the field of seismic data acquisition. More specifically, the invention relates to methods for selective bandlimited data acquisition in real time, resulting in improved imaging of economically valuable or useful earth targets of interest.
2. Background Technology
Surface acoustic sources generate seismic waves from the surface and operate at relatively low frequencies resulting in low-resolution surveys. A few downhole seismic sources have been developed which transmit acoustic waves into the formation through a borehole medium. These downhole sources can operate at a higher frequency than surface sources but often do not generate enough energy to result in accurate surveys.
Conventional downhole sources include:
the cylindrical bender source using piezoelectric rings bonded to a tube developed by Southeast Research Institute and described in Balogh et al.'s, “New Piezoelectric Transducer for Hole-to-Hole Seismic Applications,” 58th Annual International Meeting of the Society of Exploration Geophysics (1988), Session DEV2.5; the electro-acoustic transducer cylindrical bender source developed by Kompanek and described in U.S. Pat. No. 4,651,044; the arc discharge pulse source developed by Southwestern Research Institute as described in U.S. Pat. No. 5,228,011; the swept frequency borehole source developed by Western Atlas and described in Owen et al.'s, “Arc Discharge Pulse Source for Borehole Seismic Applications,” 58th Annual International Meeting of the Society of Exploration Geophysics (1988), Session DEV2.4; the potential energy “drop mass” source developed by Institut Francais du Petrole (IFP) as described in U.S. Pat. No. 4,505,362; the hammer launched source developed by OYO Corporation and described in Kennedy et al.'s “A Swept-Frequency Borehole Source for Inverse VSP and Cross-Borehole Surveying,” 7.sup.th Geophysical Conference of the Australian Society of Exploration Geophysics (1989), Volume 20, pages 133-136; and the orbital vibrator developed by Conoco and described in Ziolkowksi et al.'s “Determination of Tube-Wave to Body-Wave Ratio for Conoco Borehole Orbital Source,” 69th Annual International Meeting of the Society of Exploration Geophysics (1999), pages 156-159.
A few sources have been developed that are clamped against the borehole wall. These sources are generally more difficult to design, and not many have been developed. One source utilizes a hydraulic vibrator clamped against the borehole wall to oscillate a reaction mass axially or radially and is more fully described in Turpening et al.'s “Imaging with Reverse Vertical Seismic Profiles Using a Downhole, Hydraulic, Axial Vibrator,” 62nd Meeting of EAGE (2000), Session P0161.
Some of the most recent and promising techniques for improved imaging use complicated mathematics, such as Fourier transforms, to deconstruct the seismic data into discrete frequencies. As is well known, a Fourier transform utilizes windows, which suffer from the time-resolution frequency-localization tradeoff.
When the harmonic frequency of the desired target of interest is displayed, the image becomes much clearer than the broadband seismic image. This resonance effect is described in The Leading Edge, Interpretational Applications of Spectral Decomposition in Reservoir Characterization, Greg Partyka, 1999.
Recently, the more advanced technique of using wavelet transforms, which mitigate the significant windowing problems associated with Fourier transforms, has been successfully applied to geophysical problems.
Patent application 20050010366 of John Castagna describes the technique of Instantaneous Spectral Analysis, which decomposes the seismic signal from the time domain to the frequency domain by superimposing members of a preselected “wavelet dictionary” onto the trace, cross-correlating, and subtracting the energy of the wavelets until some predefined minimum threshold is reached. The result is a spectrum for each time location on the trace. More on this subject can be found in “The Leading Edge: Instantaneous Spectral Analysis”, John Castagna, 2003. Partyka, G. A., Gridley, J. A., and Lopez, J. A., 1999, Interpretational aspects of spectral decomposition in reservoir characterization: The Leading Edge, 18, 353-360. Castagna, J. P., Sun, Shenjie, and Siegfried, R. W., 2003, Instantaneous spectral analysis: Detection of low-frequency shadows associated with hydrocarbons, 120-127. Marfurt, K. J. and Kirlin, R. L., 2001, Narrow-band spectral analysis and thin-bed tuning: Geophysics, 66, 1274-1283. The results obtained from data deconstruction are essentially based on mathematical estimates.
Another recent industry development is time-lapse production imaging commonly termed 4D seismic monitoring. It is a method of viewing the reservoir with repeat surveys to determine how it is draining dynamically. Essentially, the seismic surveys are repeated with as much precision as possible in order to generate data sets that differ only with respect to changes associated with reservoir production. By finding the residual between the time-lapse images, one is able to infer subsurface fluid flow patterns and place constraints on fluid conduits and baffles associated with the drainage, thereby enabling one to modify reservoir models and future drilling plans. Because these changes can be subtle, any improvement of the signal-to-noise ratio would have a beneficial effect for 4D monitoring.
Accurate repositioning of the seismic source is considered one of the most critical elements in achieving 4D monitoring precision. Furthermore, a priori knowledge of the source signature would be beneficial. More on the subject can be found in the book “4D Reservoir Monitoring and Characterization” by Dr. Rodney Calvert.
The frequency range that is providing a given image is governed by the seismic wavelet, which initially represents the source signature and then changes as it experiences a number of earth-filtering effects, including absorption, geometrical spreading, and scattering. Better knowledge of the seismic source improves processes that remove the earth filtering effects.
Additional Related prior art can be found in the following:
6,985,815January 2006Castagna et al,6,661,737December 2003Wisniewski et al,5,093,811March 1992Mellor et al,6,619,394September 2003Soliman et al,200,200,700,17June 2002Soliman et al,5,077,697December 1991Chang,5,418,335May 1995Winbow,5,371,330December 1994Winbow,200,500,757,90April 2005Taner, M. Turhan et al,6,814,141November 2004Huh et al,200,201,486,06October 2002Zheng, Shunfeng et al,200,201,793,64December 2002Bussear, Terry R et al,
Variable frequency seismic sources
U.S. Pat. Nos. 4,014,403, 4,049,077, 4,410,062, 4,483,411 and 4,578,784 issued to Joseph F. Mifsud describe tunable frequency land and marine seismic vibrators.
U.S. Pat. No. 4,014,403 relates to a vibrator in which the frequency of vibration changes as the stiffness of a spring is automatically adjusted. As a result, the impedance of the spring resonates with the impedance of the reaction mass to maximize the reaction impedance, thereby increasing the operating efficiency of the vibrator.
U.S. Pat. No. 4,049,077 shows the use of a coupling plate as feedback for controlling the vibrator operation. At low frequencies, the feedback is proportional to the coupling plate position, and at higher frequencies, the feedback is proportional to the coupling plate velocity.
U.S. Pat. No. 4,410,062 shows a compliant member whose compliance is such that it is substantially rigid at the natural frequency of the vibrator, and the natural frequency of the driven load of the vibrator is within the seismic spectrum but is higher than the natural frequency of the vibrator.
U.S. Pat. No. 4,483,411 shows a seismic source, which produces a varying FM signal at the low end of the acoustic spectrum. The seismic source uses stiff oscillating radiators to create a signal in the water. These radiators are attached to devices acting as springs with a variable spring rate. Variation of the spring rate as a function of the frequency permits the device to be tuned for maximum power output.
U.S. Pat. No. 4,578,784 shows a seismic source, which produces a varying FM signal generally within the 10-100 Hz region of the spectrum.
U.S. Pat. No. 5,146,432 describes a method of characterizing transducers, and the use of a characterized transducer in the measurement of the impedance of cement located behind a section of a casing in a borehole.
U.S. Pat. No. 6,928,030 describes a seismic defense system having a closely monitored seismic source used to relay vital information from the source to the receiver.
U.S. Pat. No. 6,661,737 describes a tool including a programmable acoustic source that is controlled by a computer. The tool is used for logging.
Resonance
U.S. Pat. No. 5,093,811 refers to a fracture study technique in which resonance is established in the borehole to investigate fracture dimension by comparing the standing wave response at the wellhead to the modeled response.
U.S. Pat. Nos. 5,137,109 and 6,394,221 refer to seismic sources that sweep through a range of frequencies, the first utilizing hydraulic pressure to vary the resonance frequency of the device itself, and the second utilizing a series of variable frequency impacts to sweep the seismic range. Both are concerned with the seismic source itself and not with adjusting the source output to reach a resonant frequency of the target of interest.
U.S. Pat. No. 5,239,514 refers to a tool having frequencies in the 500-1500 Hz range, equivalent to a seismic band of 10-30 Hz, which includes much of the typical seismic band. Longer source intervals and stacking are used to increase energy and the signal-to-noise ratio. This tool does not adjust the source output to reach a resonant frequency of the target of interest in order to increase the signal-to-noise ratio, nor does it use multiple narrowbands.
U.S. Pat. Nos. 4,671,379 and 4,834,210 describe a tool that creates a standing resonant pressure wave whose frequency depends on the spacing between two end means in a borehole. Frictional, structural, and radiated acoustic energy loses are compensated for by continued application of pressure oscillations. This tool relies on establishing resonance at the source and not at the target of interest.
U.S. Pat. No. 5,081,613 describes a method that generates pressure oscillations that produce resonant frequencies in the wellbore. After removing the effects of known reflectors, the resonant frequencies are used to determine the depth and impedance of downhole obstructions. While this method does take advantage of resonance, it is confined to the wellbore and not to the target of interest.
As attested by the above references, the geophysical industry has struggled, and continues to struggle, to develop improved data acquisition techniques for improved imaging, as well as for better and easier characterization of targets of interest that are economically suitable for production, and for guidance in selecting optimum well locations with reduced investments.